Update Dockerfile to use "backend" instead of "api"

[Maximiliano Arias]

Building the back-end with Docker is failing now because the service "api" does not exist

To Reproduce
Steps to reproduce the behaviour:

1. Follow the backend installation steps with Docker

Expected behaviour
The backend builds correctly, currently it's failing:

api_1       | Error: While importing "api.run", an ImportError was raised:
api_1       |
api_1       | Traceback (most recent call last):
api_1       |   File "/usr/local/lib/python3.7/site-packages/flask/cli.py", line 235, in locate_app
api_1       |     __import__(module_name)
api_1       |   File "/badgeyay/api/__init__.py", line 5, in <module>
api_1       |     from backend.config import config
api_1       | ModuleNotFoundError: No module named 'backend'